Sputter coating is a process carried out in a vacuum chamber which is filled with a generally chemically inert gas in which a substrate is coated with a material from a target of sputtering material subjected to a negative electrical potential with respect to the chamber wall or other anode. The potential gradient adjacent the target surface causes electrons to be emitted from the target which, on their way to the chamber anode which is usually formed in part by the grounded chamber wall, strike and ionize some of the inert gas. The positive ions formed are then attracted to the negative target which they strike, transferring momentum to the target material, and ejecting particles of the material from the target surface. The substrate to be coated, which is positioned in the chamber usually with its surface facing the target, receives some of the ejected particles which adhere to and coat the substrate surface.
With magnetron sputtering, a magnetic field is formed over the target surface, usually including magnetic field lines parallel to the target surface, and, in many applications, in the form of a closed magnetic tunnel. The magnetic field causes the electrons emitted to move in curved spiral paths which trap them in regions proximate the target surface enclosed by the field, thereby increasing the rate of electron collisions with gas atoms, which in turn increase the ionization of the gas and the efficiency of the sputtering process.
In the commonly assigned and copending U.S. patent application Ser. No. 07/339,308, filed Apr. 17, 1989, entitled "Method and Apparatus for Sputter Coating Stepped Wafers", now U.S. Pat. No. 4,957,605, expressly incorporated herein by reference, a sputter coating apparatus and method are disclosed in which a concave annular target is provided with concentric annular electromagnets which cause the formation of a pair of concentric plasma rings. The plasma rings are alternately energized by alternately supplying current to energize the magnet coils while the target power level is switched in synchronization with the switching of the current to the magnetic coils. This causes different rates of sputtering from inner and outer concentric regions of the target surface, with the sputtering from each region causing different distribution characteristics of the sputtered material deposited on the substrate or wafer being coated. Varying the relative parameters affecting the energization of the two target regions provides control of coating uniformity on the substrate surfaces, which is especially important on the differently facing surfaces of stepped semiconductor wafers. The aforereferenced patent application particularly describes effects on the coating caused by the target geometry and by the electrical parameters relating to the energization of the target and plasmas.
In magnetron sputter coating processes, the sputtering of materials from the sputtering target occurs most rapidly into regions of the target where the plasma trapped by the magnetic field is the most dense. This causes the proportionate consumption or erosion of the sputtering material from the target surface. The erosion of sputtering material from other portions of the sputtering target surface generally occurs at a rate which varies in proportion to the strength and/or duration of the plasma over that portion of the target surface.
In the prior art it has been proposed in certain applications to move the magnetic field in relation to the sputtering target surface either by movement of the target or movement of the magnetic field. A purpose of the relative movement of the target or magnetic field with respect to each other is, in many cases, to provide a more uniform erosion or consumption of the sputtering target material over the surface of the target. Such devices have for many reasons been unsatisfactory.
In sputtering from a sputtering target while moving the target with respect to the magnetic field, a desirable erosion pattern is sometimes achieved for purposes of uniformly consuming the target material, but often such a pattern does not provide the proper or desired distribution of sputter coating material onto the surface of the substrate being coated. Furthermore, such devices of the prior art have insufficiently controlled the distribution of the plasma or the duration of the moving plasma with respect to the target surface so as to affect a desired non-uniform erosion pattern.
In addition, rotating magnet devices of the prior art have not effectively provided for the sputtering of the entire surface of the target. It has been found that the absence of at least some sputtering from any given region of the target may cause redeposition of the material sputtering from elsewhere on the target onto those regions where no sputtering is occurring. This causes a build-up of sputtering material which is undesirable.
Accordingly, there is a need to provide a method and apparatus for sputter coating substrates which employs a magnet which is movable relative to the sputtering target and which is capable of precisely controlling the distribution of sputtering on the target surface in its entirety.
When the magnet structure and target are rotated relative to each other, the prior art devices have failed to provide for sufficient sputtering from certain regions of the target surface, such as the center and edge regions of the target, and further have failed to effectively distribute the sputtering across the target surface in a manner which is effective to produce the desired erosion pattern to yield the proper coating uniformity on the substrate.